Lithography systems are commonly used to transfer images from a reticle onto a semiconductor wafer during semiconductor processing. A typical lithography system includes an optical assembly, a reticle stage for holding a reticle defining a pattern, a wafer stage assembly that positions a semiconductor wafer, and a measurement system that precisely monitors the position of the reticle and the wafer. During operation, an image defined by the reticle is projected by the optical assembly onto the wafer. The projected image is typically the size of one or more die on the wafer. After an exposure, the wafer stage assembly moves the wafer and then another exposure takes place. This process is repeated until all the die on the wafer are exposed. The wafer is then removed and a new wafer is exchanged in its place.
Immersion lithography systems utilize a layer of immersion fluid that completely fills a space between the optical assembly and the wafer during the exposure of the wafer. The optic properties of the immersion fluid, along with the optical assembly, allow the projection of smaller feature sizes than is currently possible using standard optical lithography. For example, immersion lithography is currently being considered for next generation semiconductor technologies including those beyond 45 nanometers. Immersion lithography therefore represents a significant technological breakthrough that enables the continued use of optical lithography.
After a wafer is exposed, it is removed and exchanged with a new wafer. As contemplated in some immersion systems, the immersion fluid would be removed from the space and then replenished after the wafer is exchanged. More specifically, when a wafer is to be exchanged, the fluid supply to the space is turned off, the fluid is removed from the space (i.e., by vacuum), the old wafer is removed, a new wafer is aligned and placed under the optical assembly, and then the space is re-filled with fresh immersion fluid. Once all of the above steps are complete, exposure of the new wafer can begin. In a tandem (or twin) stage immersion lithography system, a pair of wafer stages are provided, with the stages being alternately positioned under the optical assembly while wafer exchange and/or alignment is performed on the wafer stage not disposed under the optical assembly. When the exposure of the wafer under the optical assembly is complete, the two stages are swapped and the process is repeated. Examples of such exposure apparatus are disclosed in U.S. Pat. No. 6,341,007 and in U.S. Pat. No. 6,262,796, the disclosures of which are incorporated herein by reference in their entireties.
Wafer exchange with immersion lithography as described above continues to be problematic for a number of reasons. The repeated filling and draining of the space may cause bubbles to form within the immersion fluid. Bubbles may interfere with the projection of the image on the reticle onto the wafer, thereby reducing yields. The overall process also involves many steps and is time consuming, which reduces the overall throughput of the machine.
For examples of systems which reduce the overall throughput of the machine, see US 2006/0023186 A1 and US 2005/0036121 A1, the disclosures of which are incorporated herein by reference in their entireties.